The British Lung Foundation carried out a survey of 1,000 adults and found a third wrongly believed cannabis did not harm health.

And 88% incorrectly thought tobacco cigarettes were more harmful than cannabis ones - when the risk of lung cancer is actually 20 times higher.

Let's be clear here. It is perfectly plausible that smoking cannabis could cause lung cancer and it is perfectly plausible that joints could be more hazardous than cigarettes. They are unfiltered, for one thing, and they tend to be drawn into the lungs for longer. They are often mixed with tobacco, but even when they aren't they contain 'thousands of chemicals' just as cigarettes do (and as any burnt material will do). Any heavy dose of smoke drawn regularly into the lungs over a period of years is likely to cause damage and thus raise the risk of cancer.

The difference is that, with the exception of serious Rastafarians, people tend not to smoke twenty spliffs a day for several decades, and that is the point at which cigarette smoking becomes a serious risk.

It's a fairly small case-control study involving 79 lung cancer patients. The only unusual aspect of it is that it looks at relatively young people (55 years old or less) who have a lower cancer risk than the elderly. It gives some quite typical findings in relation to cigarette smoking. 89% of the lung cancer cases were smokers. The overall relative risk of smoking was 6.7 (3.1-14.0) and there was a clear dose-response relationship going from 2.5 amongst the light smokers up to 23.9 for the heavy smokers. The key findings are shown below—click to enlarge.

Ho-hum. And, I must add, ho-hum to another finding that is briefly mentioned which, contrary to a wealth of propaganda which says different, is quite typical of epidemiological studies on the subject...

There was no significant association between lung cancer risk and passive smoking, diet, occupation, income, educational level and alcohol use after adjustment for age, sex, ethnicity, cigarette smoking and a family history of lung cancer.

For the pot smokers, the relative risk was just 1.2 (0.5-2.6), which is insignificant in both statistical and practical terms, but for the heaviest stoners the risk was 5.7 (1.3-20.4). This is statistically significant but, as the confidence interval suggests, not totally compelling.

The groups were divided by 'pack-years' and 'joint-years'. Pack-years are a standard measure in smoking studies which combine duration of smoking with daily smoking rates. One pack a day for a year is one pack-year, half a pack for two years is one pack year, two packs for a year is two pack-years and so on.

Joint-years is a less familiar concept, but it relates to one joint smoked a day for a year. Five joints a day for a year would be five pack years. You get the picture, I trust.

What about bongs and pipes?, I hear you cry. Fear not, for the researchers had that covered...

If subjects smoked cannabis in a form other than a joint, e.g. pipes or bongs, they were asked to estimate the number of cannabis joints to which that would equate.

I will allow you, dear reader, to be the judge of how accurate you think estimates made by cannabis smokers—not renowned for their vivid memory at the best of times, and reflecting on occasions when they were off their heads on bongs and pipes—are likely to be.

Anyway, the argument used by the BLF is that the heaviest pot smokers—those who smoke a joint a day for more than 10 years (or equivalent)—have a similar lung cancer risk (RR = 5.7) to people who smoke a pack of cigarettes a day for 14 to 24 years (RR = 6.1).

Actually, they don't quite put it like that but they should have, because it's a more solid way of looking at it than what they did, which was to try to show how much risk increased with each pack/joint-year. They say (as the original researchers do) that one pack-year of smoking increases risk by 7% and one joint-year increases risk by 8%. Ipso facto, 20 cigs = one spliff.

In doing this they ignore, as so many epidemiologists do, the fact that the dose makes the poison. "Risk of developing lung cancer is up to twenty times greater in a cannabis cigarette than in a tobacco cigarette" is a silly and scientifically insupportable thing to say because it suggests that there is one joint/cigarette with your name on it. A single cigarette/joint will not give you lung cancer. It is a cumulative effect and—not incidentally—you are more likely to get it from taking up cigarette smoking because, unlike cannabis, ciggies are addictive. There really isn't much evidence that a few pack-years of smoking significantly increases lung cancer risk, but after several decades the risk rises sharply every year. If you put it on a graph it would be more of a hockey-stick than a straight diagonal line.

The same is true of pot-smoking and the research they cite clearly shows this. There is no increase in lung cancer risk for people who have less than 10.5 joint-years. One joint-year does not increase risk by 8%. On the contrary, their own data show that the light/occasional smokers of marijuana have a statistically insignificant reduction in risk of 0.3 (0.1-1.7). Those who have up to 10.5 joint-years also have a statistically insignificant reduction in risk (of 0.5 (0.1-2.0)). After that it suddenly jumps to 5.7 (1.5-21.6), albeit with only 14 cases and 4 controls involved.

This illustrates the foolishness of trying to quantify risk by individual pack-years and joint-years, let alone by individual joints, which is what today's press release essentially tried to do.

A more sensible message would be—if this one study is correct (a big if)—that chronic ganja smoking raises lung cancer risk up to the level of someone who regularly smokes for two or three decades. Equally, however, the same research could be cited as proof that you can smoke seven spliffs a week for a decade and do yourself no harm. You pays yer money and you takes yer choice, but it is irresponsible to imply that cigarettes are a safer option, just as it is irresponsible to imply that shisha smokers would be better off smoking cigarettes.

One thing I would like to know—and here I appeal to the scientists who I know read this blog—is how the researchers attributed these people's lung cancer to marijuana when 89% of them smoked cigarettes?

"...how the researchers attributed these people's lung cancer to marijuana when 89% of them smoked cigarettes?" I imagine that they carefully consulted their what's-likeliest-to-get-me-another-research-grant-ometer. Just be grateful that they didn't bang on about the synergy of marijuana and Global Warming.

Apart from anything else, I fail to see how they could have got anywhere near accurate figures for consumption of spliffs / bongs, since dope smoking tends to be a rather communal pastime, and in my experience it is rare that someone will smoke a whole spliff or a whole bong to themselves. The norm was always that it would be passed round the company present.

Given that I've smoked cigarettes for 50 years (up to 5 packs of Camel plain a day back in the 70s), and for 30 odd years I was also consuming industrial quantities of dope (my favourite mode of delivery was the chilum, probably the most potentially damaging way to smoke), my lungs, according to the pundits, should have given up the ghost years ago. Oddly, although in my 60s now, I don't even get out of breath running up the stairs. Make of that what you will.

Was looking at this study this evening and coming to similar conclusions. The most important problem you’ve identified is the problem that arises from modelling the ‘joint year’ concept as a single continuous variable, which assumes a linear function. If it turns out not to be a linear relationship, the summary statistic from the model (the 8% per joint year increase) is pretty meaningless.

For the cigarette pack years claim, a linear assumption is also made, but I think it’s probably more justifiable. To check, a means to assess if things being modelled as continuous linear variables truly are linear is to see if the model’s regression coefficients increase in a stable stepped fashion across levels of the variable.

For cigarette pack years the regression coefficients (in this case from taking the natural logs of the relevant OR) for first to fifth quintile are 0.96, 0.92, 1.81, 2.51, 3.17, resp. Hence the stepped increasing differences (second quint minus first, third minus second etc) are -0.04, 0.89, 0.70, 0.66.

So apart from the difference between the second and first quintiles, the ‘steps’ are relatively stable and a linear assumption seems sort of ok for cig pack years. At least after the second quintile.

For joint years the coefficients are (from first to third tertile are -1.20, -0.69, 1.74, giving differences of 0.51, 2.43. Clearly different as the step change in the model coefficient between 3rd and 2nd tertile are nearly 5x the difference between 2nd and 1st. A linear assumption is pretty clearly violated here and it’s surprising the authors don’t pick up on it, as it undermines the central claim to do with joint years.

This is actually just a long-winded and more boring version of what you’ve already described with the odds ratios, but it’s written now, so I’ll post it anyway!

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About Me

Writer and researcher at the Institute of Economic Affairs. Blogging in a personal capacity.
Author of Selfishness, Greed and Capitalism (2015), The Art of Suppression (2011), The Spirit Level Delusion (2010) and Velvet Glove, Iron Fist (2009).

"Of all tyrannies, a tyranny exercised for the good of its victims may be the most oppressive. It may be better to live under robber barons than under omnipotent moral busybodies. The robber baron's cruelty may sometimes sleep, his cupidity may at some point be satiated; but those who torment us for our own good will torment us without end, for they do so with the approval of their own conscience."